The present invention relates to a particulate type magnetic recording medium for high-density recording, which has a magnetic layer where a ferromagnetic powder is dispersed in a binder. More specifically, the invention relates to a magnetic recording medium for high-density recording, which has on each side thereof a magnetic layer and a substantially non-magnetic lower layer, and contains in each outermost layer a fine powder of ferromagnetic metal or hexagonal ferrite.
In the field of magnetic disks, 2MB of MF-2HD floppy disks using Cobalt-modified iron oxide have become standard with personal computers. In these days of upsurge in data volume to be processed, however, it can no longer be said that such a capacity is sufficient, but it is desirable to further enlarge the capacities of floppy disks.
In order to achieve increased recording capacity and miniaturization of recording media coupled with downsizing of computers and enhancement of computers"" ability to process information in particular, expansion of recording capacity and improvement of data transfer speed are intensely required.
Hitherto, magnetic recording media which each comprise a non-magnetic support coated with a magnetic layer containing iron oxide, cobalt-modified iron oxide, CrO2, a ferromagnetic metal powder or a hexagonal ferrite powder in a state of being dispersed in binder have been widely used. Of these magnetic substances, ferromagnetic metal powder and hexagonal ferrite powder are known to have excellent high-density recording characteristics.
In the case of disk-shaped magnetic recording media, the available large-capacity disks utilizing ferromagnetic metal powders excellent in high-density recording characteristics are 10 MB of MF-2TD and 21 MB of MF-2SD, and those utilizing hexagonal ferrite powders are 4MB of MF-2ED and 21 MB of Floptical. However, it can be said that the capacities and performances of these disks are no longer sufficient. Under these circumstances, many attempts to improve high-density recording characteristics have been made. Examples thereof are described below.
For improving characteristics of disk-shaped magnetic recording media, JP-A-64-84418 (the term xe2x80x9cJP-Axe2x80x9d as used herein means an xe2x80x9cunexamined published Japanese patent applicationxe2x80x9d) proposes using vinyl chloride resin having acidic groups, epoxy groups and hydroxyl groups, JP-B-3-12374 (the term xe2x80x9cJP-Bxe2x80x9d as used herein means an xe2x80x9cexamined Japanese patent publicationxe2x80x9d) proposes using a fine powder of metal having Hc of at least 79,600 A/m (1,000 oersted (Oe)) and a specific surface area of 25 to 70 m2/g, and JP-B-6-28106 proposes specifying the specific surface area and the magnetization intensity of a magnetic powder and further incorporating abrasives.
For improving durability of a disk-shaped magnetic recording medium, JP-B-7-85304 proposes using an unsaturated fatty acid ester and an ether linkage-containing fatty acid ester, JP-B-7-70045 proposes using a branched fatty acid ester and an ether linkage-containing fatty acid ester, JP-A-54-124716 proposes incorporating non-magnetic powder having a Mohs"" hardness of at least 6 and a higher fatty acid ester, JP-B-7-89407 discloses controlling the volume of pores taking in a lubricant to within the specified limits and the surface roughness to the range of 0.005 to 0.025 xcexcm, JP-A-61-294637 discloses the use of fatty acid esters having low and high melting points, JP-B-7-36216 discloses the use of an abrasive having a grain size equal to from one-fourth to three-fourth the magnetic layer thickness and a fatty acid ester having a low melting point, and JP-A-3-203018 discloses the use of an Al-containing ferromagnetic metal and chromium oxide.
As to the structures of disk-shaped magnetic recording media having non-magnetic lower or intermediate layers, JP-A-3-120613 proposes the structure made up of a conductive layer and a magnetic layer containing a fine powder of metal, JP-A-6-290446 proposes the structure made up of a magnetic layer having a thickness of 1 xcexcm or below and a non-magnetic layer, JP-A-62-159337 proposes the structure made up of a carbon interlayer and a magnetic layer containing a lubricant, JP-A-5-290358 proposes the structure having a non-magnetic layer in which carbon particles limited in size are incorporated, and JP-A-8-249649 proposes specifying an amount of the porosity in a lower coating layer and that in an upper magnetic layer and providing a reservoir for liquid lubricant.
On the other hand, disk-shaped magnetic recording media made up of thin-layer magnetic layers and functional non-magnetic layers have been developed in recent years, and 100 MB-class floppy disks have made their debut. As to magnetic disks having these features, JP-A-5-109061 proposes the magnetic disk constituted of a magnetic layer having Hc of at least 111,440 A/m (1,400 Oe) and a thickness of at most 0.5 xcexcm and a non-magnetic layer containing conductive particles, JP-A-5-197946 proposes the magnetic disk having a constitution that the abrasive having a size greater than the magnetic layer thickness is incorporated, JP-A-5-290354 proposes the magnetic disk having a constitution that the magnetic layer has a thickness of no greater than 0.5 xcexcm, variations in the magnetic layer thickness is controlled to within xc2x115% and the surface electric resistance is adjusted to the specified range, and JP-A-6-68453 proposes the magnetic disk having a constitution that two types of abrasives differing in grain size are incorporated and the total quantity of the abrasives at the surface are controlled to the specified range.
The reliability on magnetic disk performance, such as consistent writing and reading of data, under a condition that a magnetic disk is repeatedly used and running operations at a high speed are performed over great many times, is required more intensely than ever. For instance, JP-A-6-52541 discloses the magnetic recording medium containing as an abrasive at least one powder chosen from alumina, chromium oxide or diamond powder, and reports that the addition of such a highly hard powder has improved running stability. Further, large-capacity magnetic disks have increased linear recording density and track density, and enable a sharp decrease in area per one Bit of signal. Therefore, even minute defects on the disks come to be a fatal flaw in recording and reproducing signals.
The invention relates to a large-capacity floppy disk having improved high-density recording characteristics, and aims to provide a floppy disk having high durability and a reduced rate of error increase.
From our intensive studies to provide a magnetic recording medium having excellent electromagnetic conversion characteristics, satisfactory durability and an markedly improved error rate, especially in the high-density recording area, it has been noted that an error increase phenomenon occurred during repeated reproduction of recorded signals. By a close examination of a disk surface at which such errors occurred, it has been found that part of projections on the disk surface were shaved off by the use of a head slider and the shavings of magnetic layer adhered to other areas of the disk surface to cause defects. Further, by removal of the coating from the projections on the disk surface, it has been shown that a great part of the projections on the magnetic layer was ascribed to projections on the non-magnetic support.
More specifically, even when lower non-magnetic and upper magnetic layers provided on a non-magnetic support had a thickness several times greater than the height of projections on the non-magnetic support, the projections on the non-magnetic support formed protuberances from the magnetic layer surface. The projections on a non-magnetic support, such as a polyester film, have their source in aggregates of inorganic powder as filler, such as silica particles, and polymerization residues formed upon stretch of the polyester film.
Even the projections causing no problem in the case of using magnetic media at an ordinary number of revolutions come into high-speed collisions with a head slider; as a result, they are shaved off.
Therefore, an object of the invention is to eliminate such projections as to be shaved off by a head slider from the magnetic layer surface, thereby reducing errors in the repeated reproduction of recorded signals. This object is achieved by using any of the following magnetic recording media provided as embodiments of the invention.
1. A floppy disk-shaped magnetic recording medium which comprises a non-magnetic support provided on each side thereof sequentially with a substantially non-magnetic lower layer and a magnetic layer containing a ferromagnetic powder dispersed in a binder, and which is used under a condition that a velocity of the medium relative to a magnetic head is at least 14 m/sec: said non-magnetic lower layer having a thickness from five to twenty times as great as a maximum height of projections on the non-magnetic support surface.
2. A magnetic recording medium as described in embodiment 1, wherein the support is a film of polyester.
3. A magnetic recording medium as described in embodiment 2, wherein the polyester is polyethylene terephthalate.
When the velocity of a magnetic recording medium relative to a magnetic head is below 14 m/sec, the phenomenon in which projections are shaved off, or the issue that concerns the invention, occurs infrequently, and there is a slight increase in errors during repeated reproduction of recorded signals.
The present magnetic recording medium is a floppy disk-shaped magnetic recording medium having a non-magnetic support and, on each side of the support, a substantially non-magnetic lower layer and a magnetic layer which is provided on the lower layer and comprises a ferromagnetic powder dispersed in a binder. Compositions of layers constituting the present magnetic recording medium, layer structures the present magnetic recording medium can have, and specific methods which can be adopted for producing the present magnetic recording medium are illustrated in succession.
Magnetic Layer
It is appropriate that the magnetic layer of the present magnetic recording medium have a coercive force (Hc) of at least 143.3 kA/m (1,800 oersted), preferably at least 159.2 kA/m (2,000 oersted), particularly preferably 183.1 to 278.6 kA/m (2,300 to 3,500 oersted). When the coercive force is smaller than 143.3 kA/m (1,800 oersted), it is difficult to achieve high recording density.
Ferromagnetic Powder
The ferromagnetic powders suitable for the present upper magnetic layer are ferromagnetic metal powders and hexagonal ferrite powders.
As the ferromagnetic metal powders, ferromagnetic alloy powders containing xcex1-Fe as their main component are preferably employed.
Besides containing the atom specified above, these ferromagnetic metal powders may contain Al, Si, S, Sc, Ca, Ti, V, Cr, Cu, Y, Mo, Rh, Pd, Ag, Sn, Sb, Te, Ba, Ta, W, Re, Au, Hg, Pb, Bi, La, Ce, Pr, Nd, Sm, P, Co, Mn, Zn, Ni, Sr or/and B atoms. In particular, it is advantageous to use a ferromagnetic metal powder containing not only xcex1-Fe but also at least one atom selected from the group consisting of Al, Si, Ca, Y, Ba, La, Nd, Sm, Co, Ni and B, preferably from the group consisting of Co, Y, Al, Nd and Sm.
The suitable content of Co is from 0 to 40 atomic %, preferably from 15 to 35 atomic %, particularly preferably from 20 to 35 atomic %, based on the Fe.
The suitable content of Y is from 1.5 to 12 atomic %, preferably from 3 to 10 atomic %, particularly preferably from 4 to 9 atomic %, based on the Fe.
The suitable content of Al is from 1.5 to 30 atomic %, preferably from 5 to 20 atomic %, particularly preferably from 8 to 15 atomic %, based on the Fe.
Prior to dispersion, those ferromagnetic metal powders may be treated with a dispersing agent, a lubricant, a surfactant or/and an anti-static agent as described below. For instance, these treatments are described in JP-A-44-14090, JP-A-45-18372, JP-B-47-22062, JP-B-47-22513, JP-B-46-28466, JP-B-46-38755, JP-B-47-4286, JP-B-47-12422, JP-B-47-17284, JP-B-47-18509, JP-B-47-18573, JP-A-39-10307, JP-B-46-39639, and U.S. Pat. Nos. 3,026,215, 3,031,341, 3,100,194, 3,242,005 and 3,389,014.
The ferromagnetic metal powders may contain a small amount of hydroxides or oxides.
The ferromagnetic metal powders usable in the invention are those obtained by known production methods. The following are production methods the invention can adopt:
1) A method of reducing a compound organic acid salt (mainly an oxalate) with a reducing gas, such as hydrogen,
2) A method of producing particulate Fe or Fexe2x80x94Co via reduction of iron oxide with a reducing gas, such as hydrogen,
3) A method of thermally decomposing a metal carbonyl compound,
4) A method of adding a reducing agent, such as sodium borohydride, a hydrophosphite or hydrazine, to a water solution of ferromagnetic metals, and
5) A method of evaporating a metal in an inert gas atmosphere of low pressure, thereby pulverizing the metal.
The ferromagnetic metal powders produced by the methods as described above may be subjected to any of slow oxidation treatments, including a method of drying the powders after immersion in an organic solvent, a method of immersing the powders in an organic solvent and thereinto blowing an oxygen-containing gas to form an oxide layer on the particle surface and further drying the particles, and a method of forming an oxide layer on the particle surface by controlling pressure shares of oxygen gas and inert gas without using any organic solvent.
As to the specific surface area measured by BET method (SBET), the ferromagnetic powder contained in the present magnetic layer generally has its SBET value in the range of 45 to 80 m2/g, preferably in the range of 50 to 70 m2/g. It is undesirable for the ferromagnetic powder to have its SBET value outside the foregoing range, because SBET values smaller than 45 m2/g cause a noise increase and those greater than 80 m2/g make it difficult to attain satisfactory surface properties.
The crystallite size of the ferromagnetic metal powder is generally from 80 to 180 xc3x85, preferably from 100 to 180 xc3x85, particularly preferably from 110 to 175 xc3x85.
The suitable average particle length of the ferromagnetic metal powder is from 30 to 150 nm, preferably from 30 to 100 nm.
The suitable aspect ratio of the ferromagnetic metal powder is from 3 to 15, preferably from 5 to 12.
The saturation magnetization ("sgr"s) of the ferromagnetic powder is generally from 100 to 200 Axc2x7m2/kg (emu/g), preferably from 120 to 180 Axc2x7m2/kg (emu/g).
It is appropriate that the ferromagnetic powder have its water content in the range of 0.01 to 2.0 weight %. The water content in the ferromagnetic metal powder is preferably optimized depending on the kind of the binder used together. Further, it is advantageous that the pH of the ferromagnetic powder be optimized depending on the combination with the binder used. The optimal pH range is generally from 4 to 12, preferably from 6 to 10.
The ferromagnetic powder may receive surface treatment with Al, Si, P or an oxide of such an element, if desired. The proportion of such an element or its oxide to the ferromagnetic powder used for the surface treatment is generally from 0.1 to 10 weight %. This surface treatment can produce a desirable effect that the adsorption of a lubricant, such as fatty acids, can be controlled to 100 mg/m2 or below.
Cases are met with that the ferromagnetic powder used contains inorganic soluble ions, such as Na, Ca, Fe, Ni and Sr ions. Although it is preferable that the ferromagnetic powder be substantially free of such ions, they have little effect on characteristics of the magnetic layer so far as their content is 200 ppm or below.
Further, it is more advantageous to use a ferromagnetic powder having fewer pores. The suitable proportion of pores is 20 volume % or below, preferably 5 volume % or below. In addition, the ferromagnetic metal powder used in the invention may have any of acicular, rice-grain and spindle shapes as far as it meets the foregoing particle size requirements.
When the ferromagnetic powder itself has smaller SFD (switching field distribution), it can yield the better results. The appropriate value of SFD is 0.8 or below. In other words, it is preferable that the Hc distribution of ferromagnetic powder be made narrow. The SFD values below 0.8 are suitable for high-density digital magnetic recording, because they can ensure satisfactory electromagnetic conversion characteristics, high output, sharp magnetization flip and reduced peak shift. In the case of ferromagnetic metal powders, the narrow distribution of Hc can be attained by making a size distribution of geothite narrow or selecting a condition to retard sintering of geothite.
Carbon Black
The magnetic layer can contain carbon black, if desired. The carbon black usable in the magnetic layer includes furnace black for rubber use, thermal black for rubber use, carbon black for color, electrically conductive carbon black, and acetylene black.
It is appropriate that the carbon black used in the magnetic layer have its specific surface area in the range of 5 to 500 m2/g, its DBP absorptive capacity in the range of 10 to 400 ml/100 g, its average particle size in the range of 5 to 300 nm, its pH in the range of 2 to 10, its water content in the range of 0.1 to 10 weight % and its tap density in the range of 0.1 to 1 g/cc.